This quantity’s thermally-activated “creep” is modeled in Anderson–Kim theory. A “pinched” hysteresis loop is a signature of a device whose strength is the derivative of this quantity with respect to charge. The material derivative of this quantity vanishes for infinitely conducting fluids by Alfvén’s theorem. (10[1])The voltage produced in the AC Josephson effect is equal to the frequency times the quantum of this quantity, which equals h over 2e. The “pinning” of this quantity in superconductors (10[1])creates a force that allows them (10[1])to levitate magnets. This quantity can be computed as the double integral of “B dot dA.” The EMF induced in a loop is proportional to the time-derivative of this quantity, symbolized phi, by Faraday’s law. For 10 points, name this quantity that measures the density of magnetic field lines passing through a surface. ■END■